Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte membrane therebetween. The anode receives hydrogen gas and the cathode receives oxygen, normally distributed through porous materials called gas diffusion media. The hydrogen gas is catalytically disassociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus, are directed through an electric load, such as a vehicle, to perform work before being sent to the cathode.
Individual fuel cells are generally connected in series, or stacked one on top of the other, to form what is referred to as a fuel cell stack. The fuel cell stack is loaded in compression to maintain low interfacial electrical contact resistance between fuel cell plates, the gas diffusion media, and the catalyst electrodes. The interfacial contact resistance in the fuel cell stack is directly related to the compression loading. Typically, compression loads on the fuel cell plates range from about 50 to about 400 psi, and are controlled by a compression retention system.
Compression retention systems are typically designed in a manner effective to offset strains produced by membrane swelling and compressive stress relaxation in the fuel cell stack. Such systems act to minimize an over-compression of the diffusion media in the fuel cell stack, as well as maintain the stack compression and contact pressure between bipolar plates, gas DM, and catalyst layers. It is disclosed in U.S. Pat. No. 5,484,666 that conventional compression retention systems have consisted of tie rods extending through and between end plate assemblies secured with fastening nuts. Springs threaded on the tie rods and interposed between the fastening nuts and the end plates have been used to apply resilient compressive force to fuel cell stacks in the stacking direction.
There is a continuing need for a compression retention system that allows for a minimization of spring rate in relation to conventional systems by taking advantage of the available area along the sides of the fuel cell stack. Desirably, the compression retention system also serves as an electromagnetic interference (EMI) and an environmental enclosure.